Overview:

-The strong 1997-1998 El Niño event was a tipping point for the Great Lakes, says University of Michigan researcher.
-The Great Lakes are experiencing extremes at both ends of the thermometer, according to new study.
-"From the fish perspective, if you like to live at 20 degrees and suddenly the temperature dropped to 10 degrees, you’re still gonna die," says U of M's Hazem Abdelhady.

by CARLYN ZWARENSTEIN, Great Lakes Now

The Great Lakes, like the rest of the world, are dealing with a phenomenon of global heating caused mostly by fossil fuel emissions. 

In an innovative new study from the University of Michigan, the data reveals it’s not just rising average temperatures we need to worry about. Looking back at lake surface temperatures (LST) over 40 years, researchers found the Great Lakes have entered a new era of temperature extremes. 

But, according to Hazem Abdelhady, a water resource engineering researcher with the Cooperative Institute for Great Lakes Research at the University of Michigan and an author of the paper, lake surface temperature data only exists from 1995 to the present, since satellite technology became available. The researchers needed to find a way to extrapolate these measurements to say what lake surface temperatures were like going back far earlier.

Most existing data on extreme temperature events like heat waves or cold snaps only cover short periods of time, while most other temperature information focuses on seasonal or annual conditions. The research team found a clever way of compensating for the lack of satellite data describing ongoing lake surface temperature. They took the LST-specific information they had starting in 1995 and added a range of related measurements about atmospheric conditions, including information collected globally and dating all the way back to 1940. 

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Then, “we start(ed) forcing (the new model) using atmospheric data sets like temperature, shortwave radiation, wind speeds and other atmospheric datasets,” Abdelhady said. 

This globally gathered data extends the available information for the Great Lakes region from 1940 all the way to the present. It conveniently overlaps with the 1995-and-onward satellite data, allowing the team to confirm that this earlier, detailed but incomplete set of measurements accurately captures the same essential points as the more specific satellite temperature data. 

And what they found is extreme, at both ends of the thermometer.

“There are more than 100% increases, for example, for a lot of lakes in terms of heat waves and cold spells,” Abdelhady said. 

A tipping point occurred in the late 1990s when a strong 1997-98 El Niño event resulted in the Great Lakes absorbing a large amount of heat. After that, Abdelhady said, the extreme conditions in the Great Lakes, like heat waves and cold spells, more than doubled for most of the lakes.

Since then, the long-term warming trend has interacted with numerous other factors to increase LST extremes, with both ups and downs having profound but varied effects on the ecology and economies of the Great Lakes system, including its biodiversity. The Great Lakes are subject to the same crisis of collapsing populations and extinctions as is occurring globally due to the combination of invasive species, pollution and climate change.

Joseph Langan, a fish biologist who works on environmental research for the federal National Oceanic and Atmospheric Administration, noted in a video interview with Great Lakes Now that the system has features of a typical lacustrine ecosystem. For example, the absence of large, fast-moving currents. 

But, given how large and deep the lakes are, there are also features typical of marine environments. Wind-driven coastal upswellings of deep water are typical in Lake Michigan, and ocean fish like salmon have adapted to enact their entire life cycles within this system.

“We have these warm water species with shallow habitats that may be more negatively affected by a cold spell (but) we also have cold water species in the Great Lakes that may be more sensitive to a heat wave,” Langan said. “The timing is everything. Think about if a heat wave or cold spell hits right after a given species spawns, it’s all around the water column and maybe that allows for higher or lower survival.”

In many cases, it’s hard to say exactly how things might pan out. Langan explained, for example, that changing conditions for invasive species could have unpredictable effects throughout the food web. 

Another important feature of lake ecosystems is overturning, the mixing of water from top to bottom. During these so-called isothermal periods in fall and spring, even deep lakes may end up with roughly the same temperature throughout the water column, which is otherwise stratified into layers due to the difference in density between cold and warm water. 

Overturning allows nutrients and oxygen to cycle all the way down to the normally cold, dense bottom of the lakes. Global heating may result in more intense stratification. It can even result in hypoxia (inadequate oxygen) in lower layers. This could one day become a new and deadly tipping point, warned Abdelhady.

“If we start heating the lakes more and more, just imagine that we won’t have any isothermal (periods), it feels like it is always hot … which means less nutrients and less oxygen in the bottom layers. Which means that species that actually rely on these bottom layers will not survive,” said Abdelhady. 

With the next phase of their project, Abdelhady and his team aim to make predictions about how or when such extreme outcomes might occur. 

Given the complexity of the many interactions involved, Langan added in an email to Great Lakes Now: “research like that of Dr. Abdelhady and future studies on the effects of heatwaves/cold spells on Great Lakes fish can provide important information for fisheries managers, fishers, and other stakeholders.” 

The research group’s innovative model is already being used in both the United States and Canada across industries and species, some of which may be able to adapt to average warming temperatures.

“But the extremes don’t give you this opportunity to start adapting,” warned Abdelhady.

“A lot of people say, OK, cold extremes are fine. We are heating, so let’s just cool it a little bit,” he said. “Maybe, but from the fish perspective, if you like to live at 20 degrees and suddenly the temperature dropped to 10 degrees, you’re still gonna die.”

Editor’s note: This story has been updated to reflect that it was originally published by Great Lakes Now.